Primer on Collisions
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III) Excitation out of Metastable Levels of Atoms

C. Argon


Besides electron-impact excitation out of the metastable levels of helium, we have also recently looked at excitation out of the metastable levels of argon. Like helium, argon has two metastable levels.

Of the four levels in the 3p54s configuration, the J=0 1s3 and the J=2 1s5 are both metastable. We consider excitation into the next set of excited states arising from the 3p54p configuration.

Ground configuration of Argon: 1s2...3p6
Metastable Levels: 3p54s    3P0 (1s3), 3p54s    3P2 (1s5)

Excite metastable Ar(3p54s) + e- --> Ar(3p54p)

Click here for a more complete argon energy level diagram (GIF file: 950×700).

Excitaton into the 3p54p (J=3) level

Recall what we have learned so far about excitation of atoms from both the ground state, and from metastable states. For the ground state, excitation corresponding to dipole-allowed transitions have large cross sections with a broad excitation function. For excitation out of metastable states of helium, this rule did not hold, except for excitation into the lowest lying dipole-allowed state (23P).

Since excitation from the 3p54s metastable levels into the 3p54p levels coresponds to excitation into the the first set of dipole-allowed levels, we might expect to have large cross sections with broad excitation functions. As in helium, our target is a mixture of both metastable states. Unlike in helium, both metastable states have the same spin (both are triplets), hence we can not use the argument that spin-changing cross sections are small to rule out the contribution from a given metastable level. However, since we appear to be safe in using the dipole-allowed rule, we can apply the delta-J=0,±1 (J=0-/->J=0) selection rule to say that exctation of the 3p54p J=3 (2p9 in Pachen notation) level is due almost entirely from the J=2 1s5 metastable level.

The experimental results show a very large cross section with a broad excitation function as expected.

Measurement of Cross Sections for Electron Excitation out of the Metastable Levels of Argon Garrett A. Piech, John B. Boffard, Mark F. Gehrke, L. W. Anderson, and Chun C. Lin, Phys Rev. Lett. 81 (1998) 309-312.

Electron impact excitation out of the metastable levels of argon into the 3p54p J=3 level John B Boffard, Garrett A Piech, Mark F Gehrke, Mark E Lagus, L W Anderson, and Chun C Lin, J. Phys. B: At. Mol. Opt. Phys. 29 (1996) L795-L800.
last updated: May-19-1997